The effect of Mg–Ti deoxidation on the solidification structure of advanced high strength steel was investigated by observing the solidification structure and also the inclusion particles. The effect of precipitation of TiN, MgO, and “TiN–MgO” hybrid inclusion on the formation of fine equiaxed crystals was evaluated. The composition of inclusions was observed to change in the order of MgO → “MgO(core) + TiN(surface)” → Ti2O3 by reaction time, which corresponded to the change of solidification structure as “columnar → equiaxed → columnar.” This could be understood from the concept of lattice disregistry in between delta iron and MgO (3.97%), TiN (3.91%), and Ti2O3 (18.9%). However, even with very low disregistry between delta iron and MgO, the MgO itself did not work as an effective catalyst, indicating that there is another criterion for determining a good catalyst. The mechanism of the formation of TiN on MgO surface was schematically described. The precipitation of TiN on MgO surface was feasible although the content of Ti and N was lower than the equilibrium solubility product for the formation of TiN. Because disregistry between TiN and MgO is very low (0.05%), the precipitation of TiN on the surface of MgO is energetically more favorable.